Published on Web 02/10/2009
Magnetically Recyclable Fe@Pt Core-Shell Nanoparticles and Their Use as
Electrocatalysts for Ammonia Borane Oxidation: The Role of Crystallinity of
the Core
Xin-Bo Zhang, Jun-Min Yan, Song Han, Hiroshi Shioyama, and Qiang Xu*
National Institute of AdVanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan
Received November 11, 2008; E-mail: [email protected]
Platinum-group metals, especially Pt, are excellent and versatile
catalysts in various important reactions but occur at very low levels
of abundance in nature.1 Therefore, further enhancement of their
utilization efficiency, catalytic activity, and recycle strategy has long
been of fundamental importance.2 Arranging noble metals as thin
shells on proper non-noble-metal cores not only greatly reduces
their usage but also could significantly modify (enhance) their
catalytic properties, as a result of the synergistic structural and
electronic effects of the two metals (via the so-called strain and
ligand effects).3 Iron is well-known for its abundance and magnetism; when it is successfully employed as the core, the obtained
catalysts naturally combine the advantages of low cost and an easy
recovery function derived from use of an external magnetic field.4
Compared with its crystallized counterpart, a metal in an amorphous
state holds many more lattice defects and thus could give birth to
distinct effects in mediating the electronic structure and/or tuning
the atomic arrangement and coordination of the outer shell.5 On
the basis of this concept, localization of Pt as a thin shell on an
amorphous Fe core could be expected to not only obviously
decrease the usage of Pt but also enhance its catalytic activity.2g
Although many studies have been focused on core-shell structured
nanoparticles (NPs), employing iron as core has been rarely
reported, to say nothing of amorphous iron. Thus, the synthesis of
Fe@Pt core-shell NPs with iron cores in different crystal states
and a comparison of their catalytic activities are of great interest.
Hydrogen represents an important alternative-energy feedstock
for both environmental and economic reasons, and when it is
combined with fuel-cell technology, very efficient energy conversion
can be achieved.6 Nowadays, proton-exchange membrane fuel cells
(PEMFCs) represent the most advanced fuel-cell technology.2c-e,6c
Although pure H2 is an ideal fuel for PEMFC systems, hydrogenstorage technologies are still on the way to matching practical
requirements.7 It is now widely admitted that using other liquid
fuels to replace hydrogen for feeding fuel-cell anodes would have
many advantages in terms of fuel-cell system simplicity, high mass
and volume densities, and safety.8 Besides its high hydrogen density
(19.6 wt % H2, which exceeds that of gasoline), ammonia borane
(NH3BH3, AB) is safe, nontoxic, chemically stable, easy to transport
in its dry state, and highly soluble in water, making it emerge as a
potential fuel for fuel cells.9 In regard to a novel fuel cell using
aqueous AB as the fuel, development of efficient and economical
catalysts is highly desired for its practical application.10
Herein, we report a facile route for synthesizing carbon-supported
Fe@Pt core-shell catalysts with Fe cores in different crystal states.
Unexpectedly, in contrast to its crystallized counterpart, iron in the
amorphous state exerts a distinct and powerful ability as a core for
the Fe@Pt NPs. The resultant NPs are far more active for AB
oxidation (up to 354%) than the commercial Pt/C catalysts.
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J. AM. CHEM. SOC. 2009, 131, 2778–2779
Figure 1. (A) TEM image of carbon-supported Fe@Pt NPs and (B) XRD
patterns of (a) Fe cores reduced by NaBH4 and NH3BH3 before Pt
replacement, (b) Fe@Pt NPs, and (c) Fe@Pt NPs after heat treatment at
823 K for 3 h under an argon atmosphere.
The carbon-supported Fe@Pt NPs were synthesized using a
sequential reduction process.11 The Fe core was first synthesized
by reduction of FeSO4 using NaBH4 in an aqueous solution
containing well-dispersed carbon in the presence or absence of AB
in air at room temperature. Second, the atoms in the outer layer of
the Fe core were sacrificed to reduce Pt2+. Under such conditions,
a Pt shell can be expected to form around the Fe core.11
Figure 1A shows a transmission electron microscopy (TEM) image
of a representative sample. It was found that the NPs are isolated from
each other with high dispersion on the carbon substrate and that the
particle size is ∼2 nm. An X-ray diffraction (XRD) study showed
that the Fe core reduced by NaBH4 in the presence of AB is in the
amorphous state [Figure 1B, trace (a)], whereas without AB the Fe
core is in a crystallized state,11 which is in good agreement with the
corresponding selected-area electron diffraction (SAED) patterns.11
This difference in crystallinity of the iron core seems to have a dramatic
effect on the catalytic activity of the Fe@Pt NPs, as discussed below.
Trace (b) in Figure 1B is the XRD pattern of the Fe@Pt NPs, in which
the new, broad peak at 39.8° can be attributed to the replacement of
Pt. Sharp diffraction peaks of the platinum shell are not detected,
possibly because the layer of platinum is too thin to make significant
peaks. This XRD data is consistent with both the core-shell structure
and phase-separated monometallic particles, because if the NPs were
in a FePt alloy phase, the diffraction peaks should lie between those
of iron and platinum. However, we can exclude the phase-separated
species from consideration on the basis of the following experiments.
(1) Annealing the Fe@Pt NPs at 823 K for 3 h under an argon
atmosphere converts the sample into FePt alloy. In contrast, this did
not occur when intimately mixed physical mixtures of Fe and Pt NPs
of the same composition were heated under the conditions described
above.11 (2) The catalytic activity of the physical mixture of the Fe
and Pt NPs is quite different from that of the core-shell NPs.11
The X-ray photoelectron spectroscopy (XPS) method was employed
to further confirm the core-shell structure.11 The XPS study (Figure
S5a) reveals that the as-made NPs include only one type of Pt species.
The Pt 4f7/2 and Pt 4f5/2 binding energies correspond to Pt0. On the
10.1021/ja808830a CCC: $40.75  2009 American Chemical Society
COMMUNICATIONS
over 100 cycles and after they were kept for three weeks in air. It
was found that there was no significant current density (catalytic
activity) decrease in either case. In addition, it should be noted
that this high-performance catalyst is magnetic and thus can be
easily recovered by an external magnetic field.11
In summary, we report a facile route for the synthesis of carbonsupported Fe@Pt core-shell NPs. Surprisingly, iron in the amorphous state proved to be a distinct and powerful core for the Ptbased core-shell-structured catalyst. The resultant Fe@Pt NPs
combine high catalytic activity, low cost, long-term stability, and
easy recovery. These encouraging findings could be expanded to
other system or reactions and could open up a new direction in
catalysis. In addition, as catalysts, these NPs could be used for
optical, magnetic, and electrical applications.
Figure 2. Comparison of catalytic activities for AB oxidation of (a) carbonsupported Fe@Pt NPs with amorphous iron cores, (b) commercial Pt, (c)
Fe@Pt NPs with crystallized iron cores, and (d) the FePt alloy. Electrolyte:
0.01 M NH3BH3/1 M NaOH; sweep rate: 10 mV s-1; rotation rate: 1600
rpm; room temperature; Pt loading: 24.6 µg cm-2. Inset: Comparison of
the mass activities at -0.7 and -0.2 V.
contrary, two types of Fe species are detected (Figure S5b): Fe0 (Fe
2p3/2 located at 706.75 eV) and oxidized Fe (Fe 2p3/2 located at 710.7
eV). These oxidized Fe species are those from the outer layer of the
iron core. In order to probe the core-shell structure of the particles,
we conducted Ar sputtering experiments. The results show that the
Fe/Pt molar ratio increases after Ar sputtering, which indicates that
more Pt atoms were etched than Fe atoms. This result is in accordance
with a core-shell structure including a core mainly made of Fe and a
shell mainly made of Pt.
To compare the activities of the two types of NPs described
above, we evaluated the electrooxidation reaction of AB. Figure 2
shows the polarization curves in a 0.01 M AB/1 M NaOH solution
obtained using a rotating-disk electrode. Unexpectedly, the Fe@Pt
NPs with the amorphous Fe cores exhibit much-enhanced catalytic
activity in terms of normalized current per unit surface area of Pt,
as determined from hydrogen adsorption (Figure 2a).11 Its maximum
current is more than 3 times (as high as 354%) larger than that of
a commercial Pt/C catalyst (Figure 2b). On the contrary, the catalytic
activity of Fe@Pt NPs with the crystallized Fe core Figure 2c) is
rather poor, even less than that of commercial Pt/C. This is because,
compared with its crystalline counterpart, iron in the amorphous
state holds many more lattice defects and thus could give birth to
distinct effects in mediating the electronic structure and/or tuning
the atomic arrangement and coordination of the outer shell. As in
a core-shell architecture, the Fe metal is confined and trapped
inside the Pt shell, and thus, the conventional bifunctional mechanism cannot be implicated because AB oxidation necessarily occurs
entirely on the Pt surface sites. For comparison, after the Fe@Pt
NPs with amorphous iron cores were annealed at 823 K for 3 h,
the obtained FePt alloy (Figure 2d) suffered an obvious decrease
in catalytic activity, to a value even lower than that for Fe@Pt
NPs with crystallized Fe cores. This indicates that for AB oxidation,
the invoked bifunctionality of the coexisting Fe and Pt on the FePt
alloy surface is inferior to the ligand and surface strain effects in
the core-shell NPs. Moreover, when mass activity was used as
the indicator of electrocatalyst quality (Figure 2 inset), only Fe@Pt
NPs with the amorphous Fe cores exhibited catalytic activity greater
than that of the commercial Pt/C catalysts. This result further
confirms the role of amorphous iron as the core in Fe@Pt
core-shell NPs used as electrocatalysts for AB oxidation.
The stability or recycle ability is of great importance for the
practical application of catalysts. In this sense, we tested the
polarization curves of the Fe@Pt NPs with amorphous iron cores
Acknowledgment. The present work was financially supported
by NEDO and AIST.
Supporting Information Available: Experimental procedures and
XRD, TEM, electrochemical active surface, stability, and recycle results
for the catalysts. This material is available free of charge via the Internet
at http://pubs.acs.org.
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